Semiconductor lasers are commonly used in optical transceivers for telecommunications and data communication networks. The lasers used in such optical transceivers are commonly of the edge-emitting type. The edge-emitting laser of an optical transceiver is commonly coupled to the fiber with an aspheric lens or other discrete optical element because the light that the laser emits is not focalized or collimated, i.e., it diverges in a cone shape as it propagates. While the use of lenses to couple edge-emitting lasers to fibers in optical transceivers works reasonably well, it would be desirable to improve transceiver manufacturing economy by minimizing the number of transceiver parts and the attendant steps needed to achieve optical alignment among them.
Edge-emitting lasers for optical transceivers are fabricated on semiconductor wafers using standard photolithographic and epitaxial methods, diced into chips, and portions of each chip coated with reflective and anti-reflective coatings. The finished chips can then be tested. It would be desirable to minimize the number of manufacturing steps as well as to enhance testability.
It has also been proposed to integrate a diffractive lens and an edge-emitting laser on the same chip. For example, U.S. Pat. No. 6,459,716 to Lo et al. discloses a device in which an edge-emitted beam produced by an edge-emitting laser is reflected by an angled surface toward a lower reflective surface that is parallel to the beam-emission direction and parallel to the chip surface, which, in turn, reflects the beam upwardly in a direction generally perpendicular to the chip surface. The upwardly reflected beam is then emitted through a diffractive lens formed in a material on the chip surface. A transceiver having such a device can be manufactured more economically than one in which a separate lens is included. Nevertheless, the device is not straightforward to fabricate due to the inclusion of a waveguide to direct the beam from the laser toward the angled surface. Also, the geometry of the device may make its optical characteristics sensitive to wafer thickness errors.
Vertical Cavity Surface Emitting Lasers (VCSELs) are often preferred by end-users because of their high coupling efficiency with optical fibers without the need to provide beam shape correction, thus reducing test/packaging costs. VCSELs, however, still have problems with regard to single-mode yield control when manufactured for very high speed operation.
Efforts have also been made in the industry to convert an edge-emitting device into a vertical-emitting device. For example, U.S. Pat. No. 7,245,645 B2 discloses one or both of the laser facets etched at 45° angles to form a 45° mirror that reflects the laser beam vertically. In this solution, however, the 45° mirror is within the laser cavity. U.S. Pat. No. 5,671,243 discloses using conventional 90° laser facets that are outside of the lasing cavity, but in the same chip there is a reflection mirror that turns the beam towards in the direction of the surface. Nevertheless, the inclusion of an etched mirror inside or outside of the laser cavity requires high quality facet etching to be performed during fabrication. Performing high quality etching presents significant reliability issues, especially when performing dry etching under high operating power due to facet damage that can occur during the dry etching process.
U.S. Pat. No. 7,450,621 to the assignee of the present application discloses a solution that overcomes many of the aforementioned difficulties. This patent discloses a semiconductor device in which a diffractive lens is integrated with an edge-emitting laser on the same chip. The diffractive lens is monolithically integrated with the edge-emitting laser on an indium phosphide (InP) substrate material. The monolithic integration of a diffractive lens on the same chip in which the edge-emitting laser is integrated requires the performance of multiple Electron Beam Lithography (EBL) exposure and dry etching processes, which increases device fabrication costs.
It would be desirable to provide a semiconductor device in which an edge-emitting laser is integrated with a diffractive or refractive lens, and which is economical to manufacture.